(1) Field of the Invention
The present invention relates to a system and a method for substantially eliminating flow noise from the signal outputs of the hydrophones of a hull mounted acoustic array.
(2) Description of the Prior Art
The signal outputs of the hydrophones on hull-mounted acoustic arrays, such as the arrays mounted on the hulls of marine vessels such as surface ships, submarines and torpedoes, are corrupted by what is commonly known as “flow noise”. This is the noise produced by turbulent flow of a fluid over the hydrophones. The turbulent flow produces pressure fluctuations such that the hydrophones of the array cannot distinguish the resulting noise from the actual propagating acoustic energy of interest. This phenomenon is sometimes referred to as “pseudosound”.
When a hydrodynamically shaped body, such as a torpedo, moves through the water, the flow over the forward or nose region is laminar. At a certain point behind the nose, called the transition region, the flow “trips” or becomes turbulent. This region is referred to as the transition region and is quite well-defined. As the flow proceeds beyond the transition region, it remains turbulent. This is known as the “fully developed” region. FIG. 1 depicts a hydrodynamically shaped torpedo body 10 with the laminar region 12, the transition region 14, and the fully developed region 16. The phenomenon just described is also true for ships and submarines.
Various methods for removing flow noise have been attempted. For example, U.S. Pat. No. 4,388,711 to Fay illustrates an apparatus for rejecting turbulence induced flow noise for towed hydrophone arrays. In this apparatus, two adjacent hydrophones in the array are spaced less than flow noise coherence distance apart forming a flow noise canceling module. The inverted acoustic/flow noise signal from the lead hydrophone of the pair is split, one side passing to the final stage of signal processing while the other side, filtered, amplified, and inverted, is added to the combined signal from the lag hydrophone which has been filtered and amplified. The acoustic signal components cancel leaving only the lead hydrophone flow noise plus the lag hydrophone flow noise which has been shifted by a factor dependent upon tow vessel speed and hydrophone spacing. This flow noise signal is then combined in an adder-inverter with the delayed feedback from the output stage of this same adder-inverter, canceling the lag hydrophone flow noise leaving only the flow noise of the lead hydrophone which is then combined with the acoustic signal plus flow noise output of the lead hydrophone. The flow noise components cancel yielding only the signal of interest, free of flow noise and undistorted by minor phase shifts.
U.S. Pat. No. 5,068,834 to Fromont relates to a method and a device for causing the signal of each hydrophone of an antenna such as a hull mounted array of hydrophones to be rid of the component due to the vibrations of a close to which the antenna is placed. To this end, a correction signal is deducted from the signal of the hydrophone. This correction signal takes account of the vibrations of the wall measured by sensors placed on the wall, in the vicinity of the antenna. The study of the circuit which enables this correction signal to be given is done by resolving the vibration of the wall into its dominant modes of deformation.
U.S. Statutory Invention Registration No. H1357 relates to an active sound cancellation system for time varying signals. In this system, a high-speed control utilizes a weighted combination of open and closed-loop inputs to provide a correction signal to a cancellation source. The cancellation source introduces a canceling acoustic wave that is equal in amplitude but 180 degrees out of phase with respect to the acoustic noise in order to cancel the acoustic noise within the system. The open-loop input is provided by a database containing a predicted, off-line model of the acoustic noise. The closed-loop input is provided by a combination of (1) an input signal generated by the input acoustic wave and feedback from the cancellation source as measured by an input sensor and (2) an error signal generated at the output of the system as measured by an error sensor. The weighted combination of the open and closed-loop inputs is chosen to minimize the error signal.
The present method of removing flow noise is to use hydrophones that have an aperture that is larger than the characteristic length or “scale” of the turbulence. In this approach. if the apertures of the individual hydrophones are larger than this scale, then the pseudosound tends to cancel over this aperture. This is sometimes referred to as “k-space filtering” or “spatial filtering”. If the apertures of the individual hydrophones are not large enough, several adjacent hydrophones can be grouped into a single effective hydrophone.
There are two disadvantages to this approach. First, the grouping of individual hydrophones may not be practical due to spatial limitations. Second, it is only partially effective, i.e. it cannot remove all of the flow noise.